Heretofore, in order to improve running properties of a magnetic recording medium, prior techniques included addition of carbon black to a magnetic layer or addition of a lubricant to the surface or the interior of the magnetic layer.
These techniques can be applied to a magnetic recording medium of the type involving a plurality of superposed magnetic layers. For instance, JP-B-U-63-19941 (the term "JP-B-U" as used herein means an "examined Japanese utility model publication") discloses a magnetic recording medium in which carbon black is added in an upper magnetic layer. However, this magnetic recording medium is low in running durability and lubricating characteristics because it does not contain a lubricant.
JP-A-54-21304 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses by way of an example in the specification a magnetic recording medium which contains carbon black having an average particle diameter of 40 m.mu. in the upper magnetic layer and lecithin as a lubricant. However, lecithin is low in lubricating performance, and the carbon black is a commonly used one. In this magnetic recording medium, a rise in tension is experienced after running of about 400 passes. The reason for this is believed that the carbon black used is the type having the usual DBP oil absorption amount (less than 100 ml/100g) and is low in the ability to retain the lubricant. Therefore, it cannot hold the lubricating performance for an extended period of long time.
A magnetic recording medium which contains a lubricant in the upper magnetic layer and carbon black only in the lower magnetic layer is disclosed in U.S. Pat. No. 4,946,740. This magnetic recording medium, however, cannot hold its lubricating ability for a long period of time. Furthermore, since the upper magnetic layer does not contain carbon black, it cannot provide a suitable surface roughness.
From this viewpoint, a magnetic recording medium has been proposed in which carbon black is incorporated in both the upper and lower magnetic layers in JP-A-64-13225. This magnetic recording medium, however, has several problems in that plasticization arises in the lower magnetic layer, no sufficient durability can be obtained, and the lubricating action of the lubricant added to the magnetic layer unit is not sufficiently exhibited.